1. Field of the Invention
The present invention relates to an amplifying element and a manufacturing method thereof, and particularly to an amplifying element that is preferably used for an amplifying device and a manufacturing method thereof.
2. Description of the Related Art
In order to perform impedance conversion and amplification of an electret condenser microphone (hereinafter referred to as ECM), a junction field effect transistor (hereinafter referred to as J-FET) or an amplifier integrated circuit element is used, for example (see Japanese Patent Application Publications Nos. 2003-243944 and Hei 5-167358, for example).
FIG. 11 is a circuit diagram showing a conventional ECM 115 and an amplifying element 110 to be connected thereto. One end of the ECM 115 is connected to a gate G of the J-FET 110 which is the amplifying element. One end of the J-FET 110 is grounded, while the other end thereof is connected to a load resistance RL. Since the ECM 115 has high output impedance, a weak output current is stored in the gate G of the J-FET 110 for impedance conversion and is inputted as an input voltage. The input voltage is amplified and then a drain current with low output impedance flows from the J-FET 110. The product of a change in the drain current and the load resistance RL is extracted as an AC component of an output voltage Vout. The sensitivity of a microphone for the ECM 115 is better as the AC component of the output voltage Vout is larger.
In addition, in place of the above J-FET 110, an amplifier integrated circuit element using C-MOS or Bi-CMOS is also well-known (see, for example, Japanese Patent Application publication No. Hei 5-167358).
The amplifier integrated circuit element has advantages that an appropriate gain of the circuit element can be selectively set by adjusting a circuit constant, and that the amplifier integrated circuit element generally produces a higher gain than the J-FET. However, the amplifier integrated circuit element has a problem of having a complicated circuit configuration and of requiring high costs.
On the other hand, it is known that the J-FET has high input impedance, causes only a small amount of low-frequency noise for small signal amplification, and is excellent in a high-frequency characteristic. In addition, the J-FET has a simpler circuit configuration and requires lower cost than the amplifier integrated circuit element.
Since the amplifier integrated circuit element amplifies an input together with input noise, S/N, which is an index of sound quality, is not improved even when the gain is changed. Additionally, the noise is generated from each of resistances and semiconductors. For this reason, having a more complicated circuit configuration than the J-FET with a simple configuration, the amplifier integrated circuit element includes a larger number of noise resources, and accordingly has a lower S/N than the J-FET, in general. Accordingly, it is a common practice to use the amplifier integrated circuit element if high sensitivity is required to be achieved, but to use the J-FET if the required sensitivity is achievable by the J-FET.
However, there is a problem that the J-FET alone cannot sufficiently amplify an output, so that the resultant gain is low. As described above, the sensitivity of the ECM microphone is influenced by the AC component of the output voltage Vout amplified by the amplifying element. Accordingly, a higher gain is desirable to improve the sensitivity of microphone.
In order to increase the gain, it is effective to increase the area (cell size) of the J-FET. However, the increase in the area of the J-FET leads to an increase in an input capacitance Cin of the J-FET.
FIG. 12 shows an equivalent circuit in an output unit of the ECM and an input unit of the J-FET. In FIG. 12, VAC is an AC output voltage at the time of releasing an output of the ECM, Cm is an internal capacitance of the ECM, and Cin is an input capacitance of the J-FET.
At this time, the output of the ECM is not released and Cin is in a loaded state. As expressed by the equation of Vin=Cm/(Cm+Cin)VAC, the input voltage Vin of the J-FET in this case becomes smaller as Cm becomes smaller or Cin becomes larger, thereby causing an input loss. Here, if the input loss is reduced, the gain can be increased. Accordingly, the increase of Cm in designing the ECM unit and the reduction of Cin in designing the amplifying element lead to an improvement in the gain.
However, in order to decrease the input capacitance Cin as described above, the area of the J-FET has to be reduced. In this case, the J-FET is capable of controlling only a reduced amount of current, thereby producing only a small gain. In other words, the gain and the input capacitance Cin have a trade-off relationship. Thus, the simple and reasonable amplifying element using the J-FET has a limitation in improving the gain.